Nearly all currently manufactured electronic products, including devices such as computers, cell phones and other consumer electronics, are constructed by attaching electronic components to substrates or boards (hereafter as substrates). Electronic components include integrated circuits, discreet active and passive devices, displays and connectors. Substrates function to hold the electronic components in place and provide electrical connections between the components with desired mechanical, thermal and electrical properties. Substrates typically include a non-conductive layer or layers combined with conductive elements that function electrically in cooperation with the electronic components. Materials which form the non-conductive layers can include crystalline materials such as silicon or sapphire, amorphous materials such as non-crystalline silicon or glass, sintered ceramic materials such as aluminum oxide, or organic materials such as FR-4, polyimide, or ABF, or combinations of the preceding. Conductors are formed on or in the substrate by processes including photolithographically depositing conductive materials such as polysilicon, aluminum or copper, depositing conductive inks using screen print or ink jet technologies, or laminating and/or patterning conductive layers on or in the substrate.
What these processes have in common is the need to interconnect conductors which may be separated by layers of insulating or nonconductive material. Electronic substrates are typically made up of conductive and nonconductive layers arranged in a planar fashion. FIG. 1 shows a schematic diagram of a multilayer substrate made up of conductive or inorganic layers 10, 12 and 14, separated by insulating or organic layers 20, which may contain one or more reinforcing layers 24. FIG. 1 also shows a via 30 drilled into the substrate with conductive plating 36 forming an electrical connection between conductive layers 10 and 12.
FIG. 2 shows a schematic diagram of a prior art laser drilling system. Laser drilling systems typically comprise a laser 102 emitting laser pulses along a laser beam path 108, beam shaping optics 124, beam steering optics 128, scan lens 130, a controller 112, and a stage 142 with motion control devices (not shown) for holding the workpiece 140 and moving it in up to six axes which include translation in three orthogonal axes (X, Y and Z) and rotation about the three axes (rho, phi and theta). The controller 112 directs the laser 102 to emit energy and then coordinates the motions of the beam steering optics and the stage to position the workpiece in the appropriate place at the appropriate time. The performance of a laser via drilling system is evaluated according to criteria including throughput and via quality. Factors that determine via quality include correct location, shape, and absence of debris. Debris is defined as material that should have been removed being left in the via or material re-deposited in the via after being previously removed by the laser drill. Drilling high quality vias with little or no debris is highly desirable because it allows good mechanical contact between the conductor and the bottom of the via and the side walls. Providing a good, textured surface of the conductive layer at the bottom of the via, free of debris or remaining organic “smear” enables good electrical contact between the bottom conductor and the plating, further improving the via quality. At the same time, it is desirable to maintain as high a system throughput as possible, meaning that as little time as possible should be taken to drill a via. Via are typically drilled using pulsed laser output. For a given pulse repetition rate, this usually means drilling the via with as few pulses as possible consistent with desired quality. And finally, it is desirable to deliver a system and method to accomplish the above at a reasonable cost and complexity.
U.S. Pat. No. 6,479,788 of Arai, et. al, assigned to Hitachi Via Mechanics, Ltd. has a stated purpose of solving this problem by slicing out a series of laser pulses with decreasing pulse width of what appear to be substantially square pulses out from a CO2 laser pulse of a long pulse duration. Slicing out a series of pulses with increasingly shorter pulses out from a laser pulse with a long pulse width as is disclosed in the '788 patent is an attempt to increase the power available to micromachine the substrate while limiting unwanted thermal effects of the laser processing with long pulse width laser pulse. FIG. 3 shows exemplary laser pulses 150 of this type. However, since all the pulses have substantially the same constant peak power and square shape during the process of via drilling, it fails to address the issue of using an optimized laser pulse power profile or intensity profile to ensure the best processing results in different stages of the via drill, such as volume material rejection at the beginning of drilling and delicate cleaning of the via bottom cleaning at the end of the via drill.
Accordingly, there is a continuing need for an apparatus for laser drilling vias in electronic assemblies, capable of forming relatively debris-free, high quality vias while avoiding damage to the substrate or its surrounding structure material and maintaining acceptable system throughput.